Biofilms are formed when colonies of bacteria aggregate on surfaces in many different locations. When bacteria in biofilm aggregate, they produce a sugary, polysaccharide-containing mucous coating, or slime. Bacteria grow and multiply faster when attached (sessile) than when free-floating (planktonic). Within the slime, the bacteria form complex communities with intricate architecture including columns, water channels, and mushroomlike towers. These structural details are believed to improve biofilm nutrient uptake and waste elimination, as blood vessels do in an animal's body. More information about biofilms is provided in an article entitled “Sticky Situations: Scientists are Beginning to Understand How Bacteria Find Strength in Numbers” by Jessa Netting, published in Science News, 60:28–30, Jul. 14, 2001, which is the basis for the information in this and the following paragraphs, and which is hereby incorporated by reference herein in its entirety.
Biofilms occur in a wide range of locations. Many are found on or in the human body, including on the teeth, gums, ears, prostate, lungs, and heart, where they are believed to be implicated in chronic infections such as gum disease, ear infections, infections of the prostate gland and heart, and lung infections in people with cystic fibrosis. Biofilms also occur in nature, such as the slime that covers river rocks, marshes, and the like. Biofilms also occur in medical equipment, such as catheters, and are a major source of hospital infections. Biofilms can also occur in areas such as contact lenses; other medical equipment, and in other industries. A primary difficulty with biofilms is that they are more difficult to reduce or eliminate than are individual bacteria. This is due to the formation of the protective layer of slime, as well as adaptations that the individual bacteria undergo when they form biofilms.
One important area in which biofilms occur is in aqueous systems that use separation membranes, such as particle filtration, microfiltration, ultrafiltration, nanofiltration, and particularly reverse osmosis (“RO”) systems. Microfiltration membranes are typically polymer or metal membrane disc or pleated cartridge filters rated in the 0.1 to 2 micron range that operate in the 1 to 25 psig pressure range. Ultrafiltration is a crossflow process that rejects contaminants (including organics, bacteria, and pyrogens) in the 10 angstrom to 0.1 micron range using operating pressure in the 10 to 100 psig range. Nanofiltration equipment removes organic compounds in the 200 to 1,000 molecular weight range, rejecting selected salts. Reverse osmosis removes virtually all organic compounds and 90 to 99% of all ions under pressure in the 200 to 1000 psig range.
These systems use membranes to selectively remove or separate extremely small substances from water and process streams in residential, commercial, and industrial applications. When biofilm is present on the membrane due to microbial growth, colloidal solids and insoluble precipitates can adhere to the sticky substance. As this combination builds, water transmission rates through the membrane are reduced and/or additional pressure must be applied to maintain the same water transmission rates. Colloidal solids, microbiological growth, and insoluble precipitates can collect on the membrane during operation. Conventional treatment methods include continuous dosing, in which a residual level of a biocidal agent is maintained within the system, or periodic cleaning and sanitization, in which the filtration system is shut down for a periodic cleaning and sanitization using biocidal agents, acids and caustics. Even with continuous dosing methods, at some point the filtration system must be shut down so that the membrane can be cleaned or replaced. This results in downtime and consequent additional operating expense. Moreover, the cleaning and biocidal agents and caustics that are conventionally used to clean and sanitize the filtration systems have the effect of degrading the filter membranes, which are typically comprised of polymers such as cellulose acetate or polyamide polymers. A number of pre-treatment processes are also available to reduce the fouling potential of the feed water being introduced to the membrane. These include various types of filtration, disinfection, and chemical treatment. Even with these methods, however, most RO treatment systems must be cleaned regularly.
Accordingly, there exists a long-felt need for improved treatment processes that can achieve reductions in biofilm, particularly in the area of aqueous systems that use separation membranes.